Catalytic converters are employed today for the purification of exhaust gases. Certain relationships of the components in the exhaust gas are required for an optimal functioning of the catalytic converters. In order to be able to appropriately set these relationships, exhaust gas probes are employed, which are provided for measuring the concentration of a certain component in the exhaust gas. In particular oxygen probes, so-called lambda probes, are employed for this purpose.
Oxygen probes basically have an ionic conductive, solid electrolyte lying between two electrodes. Both electrodes can be impressed with a measuring voltage. Depending on the oxygen content and the gas to be measured, a limit current, respectively a Nernst voltage, arises which is a function of the difference of the oxygen concentrations at the electrodes.
In order to achieve the necessary ionic conductivity of the solid electrolyte, a certain operating temperature of the exhaust gas probe is required. Moreover, the measuring accuracy depends on the temperature of the probe. For this reason, it is generally required to heat the probe and to control the temperature and if need be to control said temperature in a closed loop. A separate thermocouple for measuring the temperature can be eliminated as a rule. The highly temperature-dependent internal resistance Ri of the exhaust gas probe can be accessed in order to obtain a measuring signal for the sensor temperature.
In the case of a pure 2-point lambda probe, a pulsed Ri measurement is conventionally used. In so doing, in a low-frequency range, for example smaller than 1 Hz, a current pulse of 0.5 mA to 1 mA is applied for the duration of 1-10 ms as a load to the probe for diagnostic purposes. Approximately 1 ms after switching on the current, the increase in voltage is measured. The internal resistance value measured in this way is sufficiently stable and accurate to allow for an assertion about the temperature of the probe. This method has, however, the disadvantage that a polarization of the probe is induced each time the probe is impressed with a current pulse for diagnostic purposes. This polarization has to first degrade before the probe is again available for a measurement of the Nernst voltage as an exhaust gas measurement. Particularly in the case of probes, which are aged or have cooled down, relatively long recovery phases are required for this purpose before the probe is again available for the exhaust gas measurement. This recovery phase can in particular require 20-30 ms.
In the case of continuous-action lambda probes, in particular in the case of broadband lambda probes, the Nernst voltage is constantly adjusted in a closed-loop. The controlled variable of this closed-loop control, the pump current, is the output signal of the exhaust gas probe and is used as the continuous measured value for the air ratio lambda. The temperature of the probe is also generally adjusted to a nominal value because the probe has to be operated in a narrow temperature range in order to guarantee the functionality and characteristics of the probe. A temperature measurement is required to adjust the temperature. As described at the beginning of the application, the highly temperature-dependent internal resistance Ri of the probe is generally used for this purpose. Because in the case of a continuous-action lambda probe the temperature is to be adjusted very quickly and accurately around the nominal value, a temperature measurement with high frequency, particularly with a repetition rate of at least 10 Hz, has to be implemented. The use of a pulsed measuring method greatly limits the continuous measuring of the lambda probe because a closed-loop control of the Nernst voltage is not possible during the polarization of the probe accompanying the test pulses. No measured value can therefore be acquired for O2.
In order to avoid this disadvantage, the internal resistance measurement of the probe is conventionally performed by impressing an alternating current on the Nernst cell while the voltage, which thereby arises, is accordingly evaluated. The required absence of a dc component is, for example, assured by a capacitive decoupling while the alternating current is impressed on the Nernst cell via a capacitor. In the case of probes with a pumped O2 reference, the Nernst cell is additionally impressed with a direct current, which is produced via a discrete power source. The alternating current has its own frequency, for example 3 KHz, which lies outside of the frequency band, wherein the Nernst voltage is controlled in a closed loop. This alternating current signal can therefore be filtered out from the control variable, which is evaluated for the measurement of O2, respectively for ascertaining the air ratio lambda. The filtering out of the disturbance induced by the applied alternating current, in particular this high frequency disturbance, is, however, only possible to a certain extent and requires a relatively high technical outlay. The German patent application publication DE 100 29 795 A1 describes, for example, a device for measuring the internal resistance of a linear lambda probe which has a voltage amplifier and a synchronous demodulator. The voltage amplification can be switched between two predefined values with the frequency of the AC voltage which is dropped across the internal resistance; and the output signal is flattened by means of a filter.